1. Field of the Invention
This invention relates to a method for the manufacture of phosphoric acid from naturally occurring phosphate rock and, particularly, to a method for obtaining high purity calcium sulphate and phosphoric acid from the phosphate rock.
2. Description of the Prior Art
A number of processes are known for the conversion of phosphate rock into phosphoric acid and other valuable phosphorus containing materials. Generally, there are based on the formation of mono calcium phosphate, CaH.sub.4 (PO.sub.4).sub.2 which is a water soluble compound. This is generally effected by treating the phosphate rock with sulfuric acid to form a mixture which consists predominantly of mono calcium phosphate and calcium sulphate.
These prior art processes make use of strong mineral acids for strong bases in some cases and the processes are usually carried out at relatively high temperatures which, in turn, require elaborate installations and costy materials for the plant equipment, such as, refractories, stainless steel, and the like. Moreover, a number of the prior art processes cannot be carried out effectively and efficiently on a continuous basis.
Additionally, in the United States wherein phosphate rock is mined primarily by strip-mining, the excavator or the dragline deposits the ore in sumps where it is then broken up by water jets to produce a slurry. This slurry is pumped to a beneficiation plate where it undergoes a complex procedure involving wet-screening, hydroseparation, and floatation. The beneficiation process is for the purpose of concentrating the rock so that the subsequent processes will be that much more efficient.
Certain types of rock which contain an excess of calcium carbonate or organic matter are calcined after shaking, cooling and washing to remove the lime overflow.
Consequently, the beneficiation pretreatment is quite expensive and results also in the wasting of a good part of the phosphate, generally estimated at being about one third of the initial rock. One of the reasons for this is that much of the rock after treatment has such a small particle size that it cannot be separated from the slime and tailing.
Moreover, phosphate rock from various parts of the country require specific pretreatments or additional treatments depending on the type of impurities which may be contained therein, for example, iron, aluminum, magnesium etc.. Thus, depending on the source of phosphate rock, the pretreatment steps including the beneficiation must be tailored for a particular source.
Additionally, phosphate slime, a waste product from such processes presents a major pollutant, since it is difficult to treat further to recover additional phosphate values.